The present invention relates to an apparatus for the point-by-point reading of a document using a matrix of photodetector elements. It is used more particularly in analysis of documents for telescoping.
According to the prior art, represented in particular by ACTA ELECTRONICA, vol. 21, no. 1, 1978, pp. 55-79, the analysis means of telecopiers comprise a reading head constituted by a strip of photodetector elements. In general terms, for reading a standard A4 document, the strip has 1728 aligned photodetector elements making it possible to analyze one line of the document. In order to reduce the number of connections between the photodetector element and the electronic processing circuits, there is a multiplexed addressing of these photodetector elements. These addressing circuits are produced on the actual strip. FIG. 1 gives an example of such a known strip. In the special case of FIG. 1, the photodetector elements constituting the reading head 4 of the strip are photodiodes 2 aligned along axis XX, but could equally well be photoresistors, phototransistors and in general terms any element supplying a current proportional to the light radiation received. These photodetector elements are arranged in p groups of q elements. Two circuits E.sub.x and E.sub.y control switches making it possible to separately address each photodetector element. These switches are generally realized by means of MOS transistors, whose grid is connected to circuits E.sub.x and E.sub.y. Circuit E.sub.x has p connections, each making it possible to select all the photodetector elements of a particular group. Circuit E.sub.y has q connections, each making it possible to select all the photodetector elements occupying a particular rank in a group. When a photodetector element is simultaneously selected by circuit E.sub.x and by circuit E.sub.y, it is activated, i.e. it can supply a signal which is a function of the illumination received on connections S.sub.1, . . . S.sub.q of output circuit S.
The analysis of the state of each of the photodetector elements of the strip takes place in the following way. In the initial state, the p connections X.sub.1, X.sub.2, . . . X.sub.p of E.sub.x and the q connections Y.sub.1, Y.sub.2 . . . Y.sub.q of E.sub.y are in an electrical state such that all the switches are open. The first group of q photodetector elements is then selected by bringing about the appearance on connection X.sub.1 of circuit E.sub.x of an electrical state controlling the closing of switch I.sub.1. The first photodetector element of the group is then made active by bringing about the appearance on connection Y.sub.1 of circuit E.sub.y of an electrical state controlling the closing of switch I.sub.11. An electrical state then appears on connection S.sub.1 of output circuit S, which is a function of the activated photodetector element. This signal is collected and then switch I.sub.11 is opened. The same performed for each of the photodetector elements of the group. Switch I.sub.1 is then opened and then the same sequence of operations as for the first group is carried out for each of the other groups.
The main disadvantage of this MOS switch - photodiode system is the extraction of a low level signal on connections S.sub.1, S.sub.2 . . . S.sub.q of output circuit S. Thus, the closing of switches I.sub.11, I.sub.21 . . . I.sub.p1 controlled by connection Y.sub.1 of circuit E.sub.y induces on connection S.sub.1 of output circuit S a by no means negligible switching noise, which is prejudicial to the operation of said low level signal. In the same way, the closing of the switches respectively controlled by connections Y.sub.2, Y.sub.3, . . . Y.sub.q of circuit E.sub.Y induces a switching noise on the corresponding connections S.sub.2, S.sub.3 . . . S.sub.q of output circuit S.